40 adult outpatients with MDD who failed at least one adequate trial of antidepressant monotherapy will be consented in a 63-day, cross-sequential, multicenter study comprising two treatment phases of 21 days each. The time frame from consent to baseline is 7 days. Patients will receive placebo or MSA in Phase 1, and in Phase 2, participants will receive MSA. There will also be a two-week follow up visit after the completion of Phase 2.